(a) Field of the Invention
This invention relates to a stripping tower adapted to remove unreacted monomeric vinyl chloride (hereinafter abbreviated as "MVC") from a suspension or emulsion (hereinafter simply called "slurry" whether the liquid mixture is a suspension or emulsion) of a vinyl chloride resin (hereinafter referred to as "PVC" for the sake of brevity).
(b) Description of the Prior Art
PVC is produced from MVC alone or a combination of MVC and a monomer polymerizable with MVC in accordance with the suspension or emulsion polymerization process. After completion of the polymerization, unreacted MVC is removed and recovered by reducing the pressure of the resultant reaction mixture or blowing an inert gas into the reaction mixture and PVC is thereafter dried into a final product.
Upon effecting such removal and recovery of MVC, plate-type stripping towers are generally employed owing to their high efficiency. In a conventional plate-type stripping tower, a PVC slurry is supplied from a top portion of the tower and then brought into countercurrent contact with an inert gas or steam blown as a stripping medium from a bottom portion of the tower into the tower. MVC is thus removed from the PVC slurry and then discharged as a gaseous mixture containing steam from the top of the tower. The thus-discharged gaseous mixture is then processed further for the recovery of MVC.
The above stripping tower is of the plate type and the efficiency of MVC removal varies in accordance with the number of plates. Although the efficiency becomes better as the number of plates increases, use of more plates unavoidably leads to a larger tower because it is necessary to leave a certain distance or longer between each two adjacent plates. Furthermore, a variety of PVCs is now produced. Whenever production is changed from one type of PVC to another, it is required to wash the interior of the stripping tower. The stripping tower however contains some unaccessible areas for a washing fluid as it is of the plate type. Thus, it is difficult to wash the stripping tower thoroughly.
With the foregoing in view, the present inventors attempted to use a packed tower as a stripping tower and steam as a stripping medium. The attempt resulted in good efficiency of MVC removal. However, the interior of the tower was filled up with foams, whereby developing a problem that foams of the slurry overflew from the top of the tower. Due to this problem, it was difficult to continue the operation.
As a result, the present inventors came to a conclusion that use of a packed tower as a stripping tower assures good efficiency with respect to the removal of MVC but involves a problem of foam development. The present inventors have proceeded with a further investigation on the basis of the above finding and have finally led to completion of this invention.